Synthetic Aperture Radar (SAR) technology involves the usage of large arrays. Each individual array element can be controlled individual in phase and amplitude. By this purpose a set of e.g. phase delays are programmed into all antenna elements and the resulting measurement value is stored for further processing. The strength of SAR methods lies in the fact that provided the set of phases has been sufficient, any kind of beam form can be synthesized afterwards i.e. reconstructing data that would have been measured by using a specific type of antenna with a specific beam pattern. SAR has been invented to allow the radar system to track a target without any mechanically moving parts and to be able to track several targets at the same time. The number of antenna elements required for a typical SAR applications ranges from 100s to 1000s for a 2D imaging system. Using microwave frequencies, a single SAR element does not cost much and the generation, transport and distribution (and collection) of microwave signal is cheap and there are a multitude of low-loss solutions for all kinds of geometries and topologies. The situation is completely different in submillimeterradars: For submillimeter radars there is no cheap and efficient way to generate signal power, there is neither a way to efficiently transport power over several hundreds of wavelengths (waveguides at these frequencies are expensive to machine and bends are difficult to produce, cables do not work and microstrip/stripline/coplanar waveguide technologies yield only good antennas and/or have high losses but they all are no good transmission lines above 100 GHz).
EP 807 990 B1 (The Boeing Cy) states that irregular arrays are known in the state of the art for providing a way to address grating lobe problems inherent in regular arrays because irregular arrays eliminate periodicities in the element locations. Random arrays are known in the state of the art as one form of irregular array. Random arrays are limited in their ability to predictably control worst case sidelobes. When the array element location can be controlled, an algorithm may be used to determine schemes for element placement that will allow for more predictable control of worst case sidelobes. Prior art contains many examples of irregularly spaced linear arrays many of which are non-redundant, that is, no spacing between any given pair of elements is repeated. Non-redundancy provides a degree of optimality in array design with respect to controlling grating lobes.
It also states that prior art for designing irregular planar arrays is largely ad-hoc. Only a few simple examples of non-redundant planar arrays—where there is either a relatively small number of elements or a simplistic element distribution such as around the perimeter of a circle—appear to exist in prior art. Prior art appears void of non-redundant planar array design techniques for locating an arbitrary number of elements distributed throughout the array aperture (as opposed to just around the perimeter) in a controlled manner to ensure non-redundancy and circular symmetry.
It goes on to propose a planar array design substantially absent of grating lobes across a broad range of frequencies where the available number of elements is substantially less than that required to construct a regular (i.e., equally spaced element) array with inter-element spacing meeting the half-wavelength criteria typically required to avoid grating lobe contamination in source maps or projected beams. This is done by providing a planar array of sensing or transmitting elements (e.g., microphones or antennas) spaced on a variety of arc lengths and radii along a set of identical logarithmic spirals, where members of the set of spirals are uniformly spaced in angle about an origin point, having lower worst-case sidelobes and better grating lobe reduction across a broad range of frequencies than arrays with uniformly distributed elements (e.g., square or rectangular grid) or random arrays. The array is circularly symmetric and when there are an odd number of spirals, the array is non-redundant. A preferred spiral specification embodiment combines the location of array elements on concentric circles forming the geometric radial center of equal-area annuli with locations on an innermost concentric circle whose radius is independently selected to enhance the performance of the array for the highest frequencies at which it will be used. The arrays may be used for phased electromagnetic antenna arrays.
US 2007075889 shows millimeter wave holographic imaging equipment arranged to operate with fewer antenna elements, thereby greatly reducing the cost. It involves synthetic imaging using electromagnetic waves that utilizes a linear array of transmitters configured to transmit electromagnetic radiation between the frequency of 200 MHz and 1 THz, and a linear array of receivers configured to receive the reflected signal from said transmitters. At least one of the receivers is configured to receive the reflected signal from three or more transmitters, and at least one transmitter is configured to transmit a signal to an object, the reflection of which will be received by at least three receivers.